I think they know what they are talking about, but none of us including them know many critical aspects of this virus with any accuracy at all.
Infection rate and style. (R naught)
Immunity development (really key)
How many already have been infected (also important)
How many are ambulatory and contagious (really important)
Real mortality/hospitalization rates.
They are guessing at those numbers based on some initial experiences in cruise ships and the like.